The invention relates to an electrolytic reduction pot for the production of aluminum by fused salt electrolysis, wherein the said pot comprises an outer steel shell, a thermally insulating layer and a lining mainly of carbon with iron cathode bars embedded in it.
For the production of aluminum by fused salt electrolytic reduction of aluminum oxide the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell where the surface of the molten aluminum forms the actual cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. At the anodes, as a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced which reacts with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C.
The electrical energy consumed in the electrolytic process can be classified in two main categories:
production or reduction energy PA1 energy losses.
The productive part of the energy that is consumed is required in order to reduce the Al.sup.3+ cations to metallic aluminum. This productive part of the energy consumed can therefore not be lessened.
The energy losses on the other hand can be divided into various components all of which have the effect of dissipating heat losses to the surroundings. The heat produced in the electrolytic process always flows to the colder part of the pot; from there it escapes to the surroundings thus removing energy from the production process. These heat losses can be checked and must be brought to a minimum.
By using optimally suited materials for the electrical conductors the voltage drop and with that the energy losses in the electrical circuit can be reduced to a minimum.
For a long time now it has been customary to provide a thermally insulating layer in the outer steel shell in order to prevent the loss of heat through the pot or to reduce this to a low level. Usually brick made of diatomaceous earth or moler stone is employed. New moler stone materials have excellent insulating properties; they are however very sensitive to components of the electrolyte bath which penetrate the carbon lining. For this reason the insulating layer lying closest to the electrolyte bath is often made out of less temperature sensitive but poorer insulating firebrick. Since such bricks can be readily stacked on top of each other, it is possible to insulate the sidewalls and the floor of the pot without any difficulty.
It is proposed in U.S. Pat. No. 4,052,288 to grind the linings of spent reduction cells i.e. residual carbon and insulation, and then to treat this with a strong alkaline solution so that the fluorides of sodium and aluminum are removed. A binder, usually petroleum pitch, is then added to produce a paste for lining new reduction cells.
U.S. Pat. No. 4,430,187 describes a reduction pot in which at least the lower 80% of the cell floor insulation is made up of a compressed vulcanic ash layer, the rest of the insulation on the cell floor of a leakage barrier which screens the vulcanic ash from the bath components penetrating the carbon lining.
The object of the present invention is to develop an electrolytic reduction pot for the production of aluminum by the fused salt electrolytic process, in which the manufacturing costs for the thermal insulation can be significantly lowered without the quality of the pot suffering in terms of thermal insulation and useful service life.
This object is achieved by way of the invention in that at least the lower 75% of the floor insulation of the cell is a compacted layer of a granulate material from replaced electrolytic cells essentially of particle size ranging from 0.01 to 8 mm and containing the fully ground but otherwise untreated insulation layers, without carbon residues which are mechanically sorted out before grinding. The remaining 0-25% is a layer of firebrick, ground firebrick and/or smelter alumina, and the sidewalls of the steel shell are insulated solely by firebrick.